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Lesson 5.4Power
Essential Question: How do you calculate power?
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What is power?
The rate at which work is done
The rate of energy transfer by any method
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Technical Definition
• Power is a quantity that measures the rate at which work is done or energy is transformed
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How do you find power?
Power equals work divided by time interval
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Power Alternative Formula
Power equals force times speed
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How is power measured?
• The SI unit of power is the watt, W, which is one joule per second.
• The horsepower, hp, is another unit of power.
• One horsepower equals 746 watts.
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Motors & Power
• The power output for every motor is different.
• As a result, motors will do work at different rates and transfer energy at different rates.
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A 193 kg curtain needs to be raised 7.5 m, at a constant speed, in as close to 5.0 s as possible. The power ratings for three motors are listed as 1.0 kW, 3.5 kW, and 5.5 kW. Which motor is best for the job?
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Two horses pull a cart. Each exerts a force of 250.0 N at a speed of 2.0 m/s for 10.0 min.
a. Calculate the power delivered by the horses.
b. How much work is done by the two horses?
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A kg elevator carries a maximum load of 800.0 kg. A constant frictional force of N retards the elevator’s motion upward. What minimum power, in kilowatts, must the motor deliver to lift the fully loaded elevator at a constant speed of 3.00 m/s?
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A car with a mass of kg starts from rest and accelerates to a speed of 18.0 m/s in 12.0 s. Assume the force of resistance remains constant at 400.0 N during this time. What is the average power developed by the car’s engine?
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A rain cloud contains kg of water vapor. How long would it take for a 2.00 kW pump to raise the same amount of water to the cloud’s altitude, 2.00 km?
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How long does it take a 19 kW steam engine to do J of work?
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A kg car accelerates uniformly from rest to 10.0 m/s in 3.00 s.
a. What is the work done on the car in this time interval?
b. What is the power delivered by the engine in this time interval?
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A 50.0 kg student climbs 5.00 m up a rope at a constant speed. If the student’s power output is 200.0 W, how long does it take the student to climb the rope? How much work does the student do?
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A motor-driven winch pulls a 50.0 kg student 5.00 m up a rope at a constant speed of 1.25 m/s. How much power does the motor use in raising the student? How much work does the motor do on the student?
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How do you calculate power?